The ability of medical practitioners to obtain an accurate and substantially continuous assessment of a patient's cardiac performance, and to assess system blood flow of systemic (Left side of the heart) and pulmonary (right side of the heart) circulation separately is very important.
In the current medical practices and particularly in Intensive Care Units, for instance, different devices are used for measuring the right and the left functionality of the heart during the treatment of cardiac patients. Invasive methods are usually used for obtaining measurements of the hemodynamic parameters of the right side of the heart, and noninvasive methods are usually used for measuring the hemodynamic parameters of the left side of the heart.
Some of the known invasive methods for measurement of pulmonary blood flow are: the indicator dilution methods; the blood flow determination by catherization, also known as the Swan Ganz methods; and blood flow determination by simultaneous blood sampling (from a vein and an artery coincident with measurement of oxygen consumption), also known as the Fick methods. The most widely used methods are the Swan Ganz, as described, for example, in U.S. Pat. Nos. 3,915,155, 3,726,269 and 3,651,318. The invasive methods require inserting a measuring device into the patient's body, such as a catheter in the throat, and present numerous disadvantages to both patient and physician. The patient must often endure substantial pain and discomfort and the physician must perform a relatively complicated procedure which occasionally involves exposure to the risks of contact with infectious blood.
Right heart catheterization allows precise assessment of dynamics in the right side of the heart and pulmonary arterial bed. However, this technique provides only indirect information about the left side of the heart, and in the vast majority of cases, the left side of the heart is more important in diagnostics, particularly in heart disease patients.
The noninvasive methods that are currently used for measuring systematic (left ventricle) blood flow represent a major advancement, but still have significant shortcomings. Most of those methods are based on ultrasound measurements, phonocardiography, or electrical impedance in order to calculate hemodynamic parameters, which are not adequately used to obtain a precise assessment of hemodynamic parameters of separate bodily sections.
Consequently, for many years work has been underway to develop less invasive apparatus and methods for monitoring cardiac output. For example, as an alternative to catherization methods, Doppler ultrasound techniques have been adapted to measure the velocity of blood flow. The Doppler ultrasound measurements of the ascending aorta, either externally (from the suprasternal notch) or internally (from within the trachea) can be used as a measure of cardiac output. But this technique requires determining of the diameter of the vessel, its flow profile, and the angle of the ultrasound beam relative to the vessel, in order to yield accurate results.
U.S. Pat. No. 4,671,295 describes an implementation of such methods, wherein an ultrasound transducer is mounted on the tip of an endotracheal tube so that Doppler measurements of blood flow from a point (pulse wave mode) or path (continuous wave mode) along the ultrasound beam can be measured. However, this method requires carrying out multiple measurements within the blood vessel, a priori knowledge of the blood flow pattern and cross-sectional area of the vessel and the relative angulation of the blood vessel. In addition, the accuracy of the measurements is highly dependent upon the exact placement of the transducer. These drawbacks have resulted in the slow adoption of Doppler ultrasound cardiac output techniques.
There are two noninvasive methods of electrical impedance measurements, the thoracic region and whole body methods, which are also known in the art.
These methods use an electrical impedance measurement apparatus which employs two excitation electrodes, situated at two ends of the measured section, between which a low level current is passing. Two sense electrodes, situated at intermediate locations, are used for sensing the tissue impedance. The electrical current predominantly flows through materials with high conductivities, such as blood. Smaller portions of the electrical current flow through the muscles, which have an intermediate conductivity, while another portion of the electrical current flows through fat, air, and bones, the conductivity of which is significantly smaller than that of either blood or muscles. Since the value of the resistance to current flow is a function of the conductivity and of the cross-sectional area of the conducting volume, volumes having a larger cross-sectional area are necessarily of lower resistance. Such deviations of the conducting volumes causes changes in the electrical impedance measured over time and in effect introduce difficulties in correlating the measured impedance with the cardiac parameters (such as stroke volume).
The noninvasive electrical impedance methods that are used for measuring the left ventricle hemodynamic parameters have the following disadvantages:                the thoracic electrical impedance method measures both right and left ventricle blood flows together, but it is impossible to obtain separate measurements from each ventricle utilizing the prior art methods. This shortcoming decreases accuracy of the measurement of hemodynamics parameters of the left ventricle.        the whole-body electrical impedance methods (e.g., U.S. Pat. No. 5,735,284) measures a combined impedance of the aortic blood flow (Left ventricle blood flow) and the blood flow in the peripheral organs. Therefore, the electrical impedance components, which are contributed by the blood flow via peripheral organs, introduce errors to the measurements of aortic blood flow.        
For the accurate determination of the hemodynamics parameters based on electrical impedance measurement, the following problems should be resolved:
I. Electrodes Position Problems
The manner in which the electrodes are arranged on the patient's body plays an important role in increase of accuracy of hemodynamics parameter measurements. Due to various anatomical factors, the electrodes are placed over certain areas of the body in order to achieve optimal correlation between measured changes in electrical impedance and the hemodynamics parameters. Many of the electrode configurations currently in use fail to adequately take into account the paths followed by the lines of electrical potential through the thorax and thus create distortions in the hemodynamic parameters measurement.
In electrical impedance measurement devices, the excitation and sensing electrodes are placed in proximity on the patient's chest, which as results the electrical current travels along different paths and through many different tissue interfaces. It is therefore impossible to know the exact path of the electrical current through the patient's chest. Another problem with thoracic electrical impedance has been correcting the cyclical changes of the gas volume inside the lungs. The lung impedance is directly proportional to the volume of air in the lung, i.e., as air volume increases, impedance increases, which results in distortions of the ΔZ signal. The relationship between the change of the impedance value (ΔZ) and change in the volume of air in the lung is nearly linear under most circumstances and mainly depends on the electrodes' location and chest size.
Additional disadvantages of the bioimpedance measurement methods mentioned above are the problems in the accuracy of the impedance signal measurement, which are common for all the bioimpedance methods.
II. Excitation Current Distortion Problems:
To obtain an accurate measurement of the base impedance Z and of the impedance changes ΔZ, one must take the redistribution of the current into body organs that are not supposed to be measured into account.
III. Signal to Noise Ratio Problem
There is always an ongoing effort to increase the Signal to Noise Ration (SNR) of the measured impedance signal, namely, to increase the sensitivity by increasing the SNR.
Simultaneously measurement of hemodynamic parameters of different organs and sections of the living body is very important. Although there are methods, which measure hemodynamic parameters in different sections of the human body, such as described in WO 03/003920, such prior art methods measure hemodynamic parameters with less accuracy, since those methods do not take into consideration the presence of the distortions that are introduced into the measurements due to the electrical current flows of the excitation currents to the peripheral sections which are not required for the measurements. Moreover, none of the prior art methods suggests how to cancel these distortions of the measurements.
All the methods described above have not yet provided satisfactory solutions for accurately measuring the electrical impedance of distinct body sections, and for reliably obtaining the corresponding hemodynamic parameters that can be derived from such measurements.
It is an object of the present invention to provide a method and apparatus for a noninvasive multi-channel monitoring of hemodynamic parameters of distinct sections and organs of the living body.
It is another object of the present invention to provide a method and apparatus for measurement of left and right ventricle blood flows separately.
It is another object of the present invention to provide a method and apparatus for measurement of hemodynamic parameters of systemic, pulmonary and peripheral blood flows simultaneously.
It is another object of the present invention to provide a method and apparatus for a noninvasive multi-channel monitoring of hemodynamic parameters in which the distortions introduced into the measurements due to electrical currents flows through peripheral sections of the measured body are removed by considering the excitation current distortion components.
It is a further object of the present invention to provide a method and apparatus for a noninvasive multi-channel monitoring of hemodynamic parameters utilizing an electrode configuration which provides improved uniformity of field distribution within sections and organs of the measured body.
It is another object of the present invention to provide a method and apparatus for digital signal processing for increasing the accuracy and sensitivity of measurement of bioimpedance of sections and organs of measured body.
Other objects and advantages of the invention will become apparent as the description proceeds.